This invention relates to discharge lamp operating circuits, and more particularly, to high frequency circuits for efficiently operating high intensity discharge (HID) lamps.
Attempts to operate HID lamps from so called electronic ballasts often have been confronted with a problematic phenomena referred to as acoustic resonance. This phenomena causes the arc within the lamp to girate, flicker, and even to extinguish when a ballast circuit is designed to operate the lamp at frequencies greater than a few kilohertz. Of the several classes of electronic ballasts, the types that operate the HID lamps on high frequency AC tend to be the most desirable from the lamp operating point of view, if the acoustic resonance problem can be overcome. The high frequency output for operating the lamp can be generated with compact, light-weight circuits, and lamp efficiency is enhanced by the use of high frequency; for example, use U.S. Pat. No. 4,170,746. Other types of electronic ballasts using DC lamp operation or phase control at low frequencies are not as desireable from a lamp life point of view. Also, DC operation can cause segregation of additives in metal halide lamps which causes color bands to appear within the arc. Since high frequency AC operation is deemed desireable, therefore, a particularly challanging problem is overcome in this field is the aforementioned acoustic resonance phenomena.
The arc tubes employed in HID lamps are hollow tubes of alumina, quartz or hard glass shaped in various sizes with cupped or conical ends and filled with gas at several different pressures. The exact gas pressure depends on the operating state of the lamp, the pressure being comparatively low for a cold start and becoming progressively higher as a lamp warms up over a period of several minutes. "Organ Pipe" resonances can occur at different lamp operating frequencies depending on the size, shape, and pressure within the arc tube. For example, we have found that larger lamps, e.g., in the 400 and 1,000 watt category, have resonances at operating frequencies at and above 2 kilohertz, with harmonics well up into the supersonic range. Smaller lamps, e.g., 175 watts, 100 watts, etc., have resonances at higher frequencies. Operation near a resonance point on high frequency AC causes the arc to flicker, stretch, elongate and spiral. Sometimes the lamp extinguishes. Arc tube breakage due to resonance has been reported in the literature.
Heretofore, it appears that no method has been known for obviating this phenomena of acoustic resonance other than to avoid the resonance range of frequencies for a given lamp arc tube; for example, see the aforementioneds U.S. Pat. No. 4,170,746. Operation at any frequency above primary resonance, however, can also be problematical because of lamp pressure variability with temperature and the differences from lamp to lamp in arc tube shape.
Other solutions to this problem that have been proposed include placing quartz wool inside the arc tube so as to damp the resonances; however, we are aware of no practical success in this area. It has also been suggested that square wave AC operation of the lamp would overcome the problem. Although such operation has resulted in a slight reduction in the severity of the resonance problem, this approach does not provide the complete answer, possibly because it is nearly impossible to generate a power AC with a perfect square wave shape. The deviations from the perfect square wave comprise overshoots, droops, and ringing which excite the lamp into resonance in any event.